My guess is that Facebook will become a bit player in its own market, like AOL did. Social networking will become almost part of the operating systems, so a one-stop-social-network-shop will be no more desireable than a one-stop-computer-network-shop was ~9 years ago, when AOL flopped.
Originally, AOL provided access to a lot of networked content, but the internet superceded AOL's content. So AOL became an internet portal, but there are a lot of internet portals and so AOL became just another ISP.
My guess is that history will repeat itself. Lots and lots of sites are trying to incorporate social networking (from Flickr to Battle.net, sheesh) - soon (and probably already) there will be free apps that work with a variety of social networking sites, and then it will become much less important what stuff you're putting on Facebook vs MySpace etc, just like it's not terribly important to others whether your ISP is SBC, ComCast, AOL, etc.
Just my 3 cents worth (I like to think I'm 1 cent better).
ETA: I'm trying to come up with a rationale for why Google's ongoing success isn't a good counterexample. I don't think it's a good counterexample, but I'm not sure I have a good reason why not. And that, of course, means that I may be wrong.